Seasonal and diel changes in the structure of a crustacean decapod assemblage associated to a shallow Cymodocea nodosa meadow in northern Tunisia (Mediterranean Sea). An overview of the Mediterranean decapod taxocoenoses.

Περίληψη

A study on the decapod assemblage inhabiting a shallow meadow of Cymodocea nodosa from the bay of La Goulette (Tunisia) was carried from February 2009 to February 2010. Monthly samples (with replicates) were taken in the morning and at night, with a small Agassiz trawl. In total 11699 specimens belonging to 41 species were caught. Significant day-night and seasonal changes are mainly related to movements (feeding) and recruitments. Higher abundance and richness were reported at night. Analyses rendered no significant relationships between the plant phenology and abundances and richness; neither between the total abundance and temperature, but significant correlations with a time lag of 3 - 4 months between these factors exist, which could be related with hatching and larval development period. The nocturnal and diurnal values in the diversity and equitability indexes are quite similar along the year (peaks in spring - early summer, minimum in autumn - early winter). This pattern is mainly due to the strong dominance of a few species, with maximum abundances during their recruitment events, whose seasonal and day-night changes have been determined. Besides, an overview of the decapods assemblage associated to this seagrass for the Mediterranean Sea has been carried out. The species can be divided into two groups (linked to the leaves and to the sediment). The differences in species composition between areas must be related to different sampling methodologies, feeding and reproductive strategies, but also to the layout and influence of the surrounding habitats.